Influence of triphosphine ligand coordination geometry in Mn(I) hydride complexes [(PPP)(CO)MnH] on their kinetic hydricity.

Octahedral Mn(I) complexes bearing tridentate donor ligands [(LL'L'')(CO)MnX] have recently emerged as major players in catalytic (de)hydrogenation processes. While most of these systems are still based on structurally rigid pincer scaffolds imposing a meridional coordination mode, for some more flexible tridentate ligands a facial arrangement of donor moieties becomes possible. Accordingly, the geometry of the corresponding Mn(I) hydrides [(LL'L'')(CO)MnH] directly involved in the catalytic processes, namely the nature of the donor extremity located in the -position of the hydride (CO and L for - and -configurations, respectively) may influence their hydride transfer ability.

Substituents' Effect on the Photophysics of Trinuclear Copper(I) and Silver(I) Pyrazolate-Phosphine Cages.

A series of structurally similar trinuclear macrocyclic copper(I) and silver(I) pyrazolate complexes bearing various short-bite diphosphine RPCH(R')PR ligands are reported. Upon diphosphine coordination, the planar geometry of the initial complexes undergoes bending along the line between two metal atoms coordinated to the phosphorus moieties. The complexes based on dcpm ligands (R = cyclohexyl, R' = H, Ph) do not exhibit dynamic behavior in solution at room temperature on the P NMR time scale as it was previously observed for similar trinuclear copper complexes bearing the dppm (R = Ph, R' = H) scaffold.

Nitrogen Fixation by Manganese Complexes - Waiting for the Rush?

Manganese is currently experiencing a great deal of attention in homogeneous catalysis as a sustainable alternative to platinum group metals due to its abundance, affordable price and low toxicity. While homogeneous nitrogen fixation employing well-defined transition metal complexes has been an important part of coordination chemistry, manganese derivatives have been only sporadically used in this research area. In this contribution, the authors systematically cover manganese organometallic chemistry related to N activation spanning almost 60 years, identify apparent pitfalls and outline encouraging perspectives for its future development.

Impact of the Methylene Bridge Substitution in Chelating NHC-Phosphine Mn(I) Catalyst for Ketone Hydrogenation.

Systematic modification of the chelating NHC-phosphine ligand (NHC = N-heterocyclic carbene) in highly efficient ketone hydrogenation Mn(I) catalyst fac-[(PhPCHNHC)Mn(CO)Br] has been performed and the catalytic activity of the resulting complexes was evaluated using acetophenone as a benchmark substrate. While the variation of phosphine and NHC moieties led to inferior results than for a parent system, the incorporation of a phenyl substituent into the ligand methylene bridge improved catalytic performance by ca. 3 times providing maximal TON values in the range of 15000-20000.

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes intermolecular bimetallic cooperation.

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (M⋯M') have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate the synergetic action of Lewis acidic (M) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br).

Phosphine-NHC-Phosphonium Ylide Pincer Ligand: Complexation with Pd(II) and Unconventional -Coordination of the Ylide Moiety.

An efficient synthesis of two pincer preligands [PhPCH(R)ImCHCHCHPPh]X (R = H, X = OTf; R = Ph, X = BF) was developed. Subsequent reactions with PdCl and an excess of CsCO led to the formation of highly stable cationic metalated Pd(II) complexes [(,,,)Pd]X exhibiting phosphine, NHC, phosphonium ylide, and σ-aryl donor extremities. The protonation of one of the latter complexes with R = H affords the Pd(II) complex [(,,)Pd(MeCN)](OTf) bearing an unprecedented nonsymmetrical NHC core pincer scaffold with a 5,6-chelating framework.

An Experimental and Computational Investigation Rules Out Direct Nucleophilic Addition on the N Ligand in Manganese Dinitrogen Complex [Cp(CO) Mn(N )].

We have re-examined the reactivity of the manganese dinitrogen complex [Cp(CO) Mn(N )] (1, Cp=η -cyclopentadienyl, C H ) with phenylithium (PhLi). By combining experiment and density functional theory (DFT), we have found that, unlike previously reported, the direct nucleophilic attack of the carbanion onto coordinated dinitrogen does not occur. Instead, PhLi reacts with one of the CO ligands to provide an anionic acylcarbonyl dinitrogen metallate [Cp(CO)(N )MnCOPh]Li (3) that is stable only below -40 °C.

The Dichotomy of Mn-H Bond Cleavage and Kinetic Hydricity of Tricarbonyl Manganese Hydride Complexes.

Acid-base characteristics (acidity, pa, and hydricity, ΔG° or ) of metal hydride complexes could be a helpful value for forecasting their activity in various catalytic reactions. Polarity of the M-H bond may change radically at the stage of formation of a non-covalent adduct with an acidic/basic partner. This stage is responsible for subsequent hydrogen ion (hydride or proton) transfer.

Redox-Switchable Behavior of Transition-Metal Complexes Supported by Amino-Decorated N-Heterocyclic Carbenes.

The coordination chemistry of the N-heterocyclic carbene ligand IMes, derived from the well-known IMes ligand by substitution of the carbenic heterocycle with two dimethylamino groups, was investigated with d [Mn(I), Fe(II)], d [Rh(I)], and d [Cu(I)] transition-metal centers. The redox behavior of the resulting organometallic complexes was studied through a combined experimental/theoretical study, involving electrochemistry, EPR spectroscopy, and DFT calculations. While the complexes [CuCl(IMes)], [RhCl(COD)(IMes)], and [FeCp(CO) (IMes)](BF) exhibit two oxidation waves, the first oxidation wave is fully reversible but only for the first complex the second oxidation wave is reversible.

-to- isomerization triggers hydride transfer from Mn(I) complex -[(dppm)Mn(CO)H].

Low-temperature IR and NMR studies combined with DFT calculations revealed the mechanistic complexity of apparently simple reactions between Mn(I) complex -[(dppm)Mn(CO)H] and Lewis acids (LA = PhC, B(CF)) involving the formation of so-far elusive meridional hydride species -[(dppm)Mn(CO)H⋯LA] and unusual dearomatization of the PhC cation upon hydride transfer.

Carbenes and phosphonium ylides: a fruitful association in coordination chemistry.

Among a plethora of σ-donor ligands available, carbon-centered ones have become essential, in particular with the emergence of N-heterocyclic carbenes (NHCs), positioning themselves as credible alternatives to traditional nitrogen- and phosphorus-based systems. Phosphonium ylides representing another class of neutral η-bonded carbon ligands have also been shown to act as effective Lewis bases. Considering the intrinsic features of the carbene and phosphonium ylide ligands, similar in terms of electronic properties, but different in terms of bonding mode, the design of hybrid systems combining these two types of carbon functionalities appeared to be a natural and exciting challenge.

Half-sandwich manganese complexes Cp(CO)Mn(NHC) as redox-active organometallic fragments.

Oxidation of the half-sandwich Mn complexes Cp(CO)Mn(NHC) bearing dialkyl-, arylalkyl- and diarylsubstituted N-heterocyclic carbene ligands (NHC = IMe, IMeMes, IMes) affords the corresponding stable Mn radical cations [Cp(CO)Mn(NHC)](BF) isolated in 92-95% yield. Systematic X-ray diffraction studies of the series of Mn and Mn NHC complexes revealed the expected characteristic structural changes upon oxidation, namely the elongation of the Mn-CO and Mn-NHC bonds as well as the diminution of the OC-Mn-CO angle. ESR spectra of [Cp(CO)Mn(IMes)](BF) in frozen solution (CHCl/toluene 1 : 1, 70 K) allowed the identification of two conformers for this complex and their structural assignment using DFT calculations.

Direct Access to Palladium(II) Complexes Based on Anionic ,,-Phosphonium Ylide Core Pincer Ligand.

The reaction of readily available imidazolium-phosphonium salt [MesIm(CH)PPh](OTf) with PdCl in the presence of an excess of CsCO afforded selectively in one step the cationic Pd(II) complex [(,,)Pd(NCMe)](OTf) exhibiting an LX-type NHC-ylide-aryl ,,-pincer ligand via formal triple C-H bond activation. The replacement of labile MeCN in the latter by CNBu and CO fragments allowed to estimate the overall electronic properties of this phosphonium ylide core pincer scaffold incorporating three different carbon-based donor ends by IR spectroscopy, cyclic voltammetry, and molecular orbital analysis, revealing its significantly higher electron-rich character compared to the structurally close NHC core pincer system with two phosphonium ylide extremities. The pincer complex [(,,)Pd(CO)](OTf) represents a rare example of Pd(II) carbonyl species stable at room temperature and characterized by X-ray diffraction analysis.

Experimental and Theoretical Insights into the Electronic Properties of Anionic N-Heterocyclic Dicarbenes through the Rational Synthesis of Their Transition Metal Complexes.

The lithiation of the NHC ligand backbone in Cp(CO)Mn(IMes) followed by transmetalation on the C4 carbenic position with Cp(CO)FeI led to the heterobimetallic complex Cp(CO)Mn(IMes)Fe(CO)Cp bearing the anionic ditopic imidazol-2,4-diylidene IMes ligand. Subsequent treatment of the later with TfOH induced a selective decoordination of the [Cp(CO)Mn] fragment to form the cationic abnormal NHC complex [Cp(CO)Fe(IMes)](OTf), which was further derivatized to the bis(iron) IMes complex [Cp(CO)Fe(IMes)Fe(CO)Cp](OTf) by reaction with AmOK and Cp(CO)FeI. The effect of the metalation at the C4 or C2 position on the imidazole ring on the electronic donation properties of the associated C2 and C4 carbenic centers in the IMes ligand was quantified through systematic experimental and theoretical studies of IMes, IMes, and IMes complexes.

NHC Core Phosphonium Ylide-based Palladium(II) Pincer Complexes: The Second Ylide Extremity Makes the Difference.

The coordinating properties of N-heterocyclic carbene (NHC) (A), phenolate (B), and phosphonium ylide (C) moieties were investigated systematically through the preparation of a family of NHC, phosphonium ylide-based pincer ligands, where the third donor extremity can be either an NHC, a phenolate, or a phosphonium ylide. The overall donor character of such ligands [NHC(ABC)] ( + + = 2) was analyzed by comparison of the molecular orbitals (energy and shape), oxidation potentials (), and IR ν and ν stretching frequencies of their isostructural pincer Pd(II) complexes [NHC(ABC)PdL][OTf] (L = NCCH, CO, or CNBu). The three categories of pincer complexes based on phosphonium ylides were easily obtained by acidic treatment of their highly stable -metalated Pd(II) precursors prepared in a single step from readily available N-phosphonio-substituted imidazolium salts.

Bis[diphenylphosphino]methane and its bridge-substituted analogues as chemically non-innocent ligands for H activation.

Deprotonation of fac-[(κ2P,P-Ph2PCH(R)PPh2)Mn(CO)3Br] (R = H, Me, Ph) produces the corresponding diphosphinomethanide derivatives fac-[(κ3P,C,P-Ph2PC(R)PPh2)Mn(CO)3], which are prone to activate H2 to form the hydride complexes fac-[(κ2P,P-Ph2PCH(R)PPh2)Mn(CO)3H]. The substitution of the dppm bridge improves dramatically the reaction efficiency and this was rationalized by DFT calculations.

Phosphine-NHC Manganese Hydrogenation Catalyst Exhibiting a Non-Classical Metal-Ligand Cooperative H Activation Mode.

Deprotonation of the Mn NHC-phosphine complex fac-[MnBr(CO) (κ P,C-Ph PCH NHC)] (2) under a H atmosphere readily gives the hydride fac-[MnH(CO) (κ P,C-Ph PCH NHC)] (3) via the intermediacy of the highly reactive 18-e NHC-phosphinomethanide complex fac-[Mn(CO) (κ P,C,C-Ph PCHNHC)] (6 a). DFT calculations revealed that the preferred reaction mechanism involves the unsaturated 16-e mangana-substituted phosphonium ylide complex fac-[Mn(CO) (κ P,C-Ph P=CHNHC)] (6 b) as key intermediate able to activate H via a non-classical mode of metal-ligand cooperation implying a formal λ -P-λ -P phosphorus valence change. Complex 2 is shown to be one of the most efficient pre-catalysts for ketone hydrogenation in the Mn series reported to date (TON up to 6200).

Palladium(ii) pincer complexes of a C,C,C-NHC, diphosphonium bis(ylide) ligand.

A new family of pincer palladium(ii) complexes bearing an electron-rich C,C,C-NHC, diphosphonium bis(ylide) ligand of LX-type was prepared through the dual N-functionalization of 1H-imidazole by (3-bromopropyl)triphenylphosphonium bromide. Selected basic conditions allowed the sequential coordination of the NHC and phosphonium ylide moieties to Pd(ii). This strategy led to an original ortho-metallated complex where the Pd center is bonded to four carbon atoms of three different natures: carbenic (sp), arylic (sp), and chiral ylidic (sp).

Manganese catalyzed reductive amination of aldehydes using hydrogen as a reductant.

A one-pot two-step procedure was developed for the alkylation of amines via reductive amination of aldehydes using molecular dihydrogen as a reductant in the presence of a manganese pyridinyl-phosphine complex as a pre-catalyst. After the initial condensation step, the reduction of imines formed in situ is performed under mild conditions (50-100 °C) with 2 mol% of catalyst and 5 mol% of tBuOK under 50 bar of hydrogen. Excellent yields (>90%) were obtained for a large combination of aldehydes and amines (40 examples), including aliphatic aldehydes and amino-alcohols.

Oxidative Coupling of Anionic Abnormal N-Heterocyclic Carbenes: Efficient Access to Janus-Type 4,4'-Bis(2H-imidazol-2-ylidene)s.

The oxidative coupling of anionic imidazol-4-ylidenes protected at the C2 position with [MnCp(CO) ] or BH led to the corresponding 4,4'-bis(2H-imidazol-2-ylidene) complexes or adducts, in which the two carbene moieties are connected through a single C-C bond. Subsequent acidic treatment of the later species led to the corresponding 4,4'-bis(imidazolium) salts in good yields. The overall procedure offers practical access to a novel class of Janus-type bis(NHC)s.

Manganese-mediated synthesis of an NHC core non-symmetric pincer ligand and evaluation of its coordination properties.

The alkylation of N-(2-pyridyl)imidazole by the Mn methylene-phosphonium complex [Cp(CO)Mn(η-P,C-PhP[double bond, length as m-dash]C(H)Ph)]BF offers a straightforward route to a tridentate pro-ligand featuring an NHC core and phosphine/pyridine arms. The ability of this PĈN ligand to coordinate in a pincer mode was recognized in Rh, Rh, and Ni complexes.

Post-coordination backbone functionalization of an imidazol-2-ylidene and its application to synthesize heteropolymetallic complexes incorporating the ambidentate IMes(CO2(-)) ligand.

Backbone functionalization of the pre-coordinated, emblematic IMes ligand with the COOH moiety was achieved upon treatment of Cp(CO)2Mn(IMes) with n-BuLi followed by CO2 and HCl to afford Cp(CO)2Mn(IMes(COOH)). The latter reacts with (1,10-phen)M(OAc)2 (M = Cu, Zn) to obtain M3Mn4 polymetallic complexes exhibiting the ambidentate IMes(CO2(-)) moiety as a key structural unit.

Catalytic Scanning Probe Nanolithography (cSPL): Control of the AFM Parameters in Order to Achieve Sub-100-nm Spatially Resolved Epoxidation of Alkenes Grafted onto a Surface.

Scanning probe lithography (SPL) appears to be a reliable alternative to the use of masks in traditional lithography techniques as it offers the possibility of directly producing specific chemical functionalities with nanoscale spatial control. We have recently extend the range of applications of catalytic SPL (cSPL) by introducing a homogeneous catalyst immobilized on the apex of a scanning probe. Here we investigate the importance of atomic force microscopy (AFM) physical parameters (applied force, writing speed, and interline distance) on the resultant chemical activity in this cSPL methodology through the direct topographic observation of nanostructured surfaces.

Umpolung of methylenephosphonium ions in their manganese half-sandwich complexes and application to the synthesis of chiral phosphorus-containing ligand scaffolds.

Half-sandwich manganese methylenephosphonium complexes [Cp(CO)2Mn(η(2)-R2P=C(H)Ph)]BF4 were obtained in high yield through a straightforward reaction sequence involving a classical Fischer-type manganese complex and a secondary phosphine as key starting materials. The addition of various nucleophiles (Nu) to these species took place regioselectively at the double-bonded carbon center of the coordinated methylenephosphonium ligand R2P(+)=C(H)Ph to produce the corresponding chiral phosphine complexes [Cp(CO)2Mn(κ(1)-R2P-C(H)(Ph)Nu)], from which the phosphines were ultimately recovered as free entities upon simple irradiation with visible light. The synthetic potential of this umpolung approach is illustrated herein by the preparation of novel chiral pincer-type phosphine-NHC-phosphine ligand architectures.